The present invention relates to an improved method for crystallizing anhydrous fructose crystals from water solution. Disclosed herein is an economic method for producing large scale, high yields of crystalline fructose. Crystalline fructose is generally obtained by seeding supersaturated fructose solutions to induce crystalline growth. Due to the solubility and stability characteristics of fructose and high viscosity of fructose solutions, however, it is often problematic to maintain the optimum conditions to insure the economic production of a pure crystalline product.
Fructose is very soluble in water and the solutions are extremely viscous. A large amount of heat due to high crystallization heat of fructose, mixing heat and additional cooling of the mass must be removed during fructose crystallization. In addition, because fructose has a very narrow metastable zone, the temperature difference between the solution and the cooling surface must be quite low thus making the crystallization very difficult.
To overcome this difficulty, several prior art processes involve the use of organic solvents to crystallize fructose from aqueous solutions. In Finnish Patent Application No. 862025, for example, a continuous fructose crystallization method using organic solvents is described. The viscosity of the fructose solution, however, results in a lowering of productivity, thus the yield is only about 40% and the productivity about 0.17 t/m.sup.3 /d even if the mass is pumped through a vertical crystallizer. The productivity (t/m.sup.3 /d) is defined as the production rate of crystals (metric tons) per the total volume of the crystallizer (cubic meter).
Crystallization from an organic solvent or water solvent mixture is also described in Staley's European patent 015617. The use of organic solvents, however, creates disadvantages with large scale crystallizations. These include fire hazards as well as the fact that solvents are toxic and therefore unsuitable because small residues remaining in the crystalline product will leave it unsuitable for use in foods.
Several methods have been developed which avoid the use of organic solvents in the fructose crystallization process, but these methods are often disadvantageous economically because of the high viscosity and unstable nature of supersaturated fructose solutions. UK Patent Application 2172288A teaches a method for the continuous crystallization of fructose from an aqueous solution. The syrup is rapidly mixed with seed and put onto a surface until a cake is formed, which is then comminuted to a free flowing granular product. Although this method avoids the problem of continuous handling of viscous solutions, the granular amorphous product contains all of the impurities that were in the feed syrup. In addition, the extra grinding and drying stages raise the operation costs considerably. Similar costs are incurred using the method described in U.S. Pat. No. 4,199,373, wherein syrup is seeded with crystalline fructose and allowed to stand in a mold or container, after which the crystalline material is recovered, dried, and ground.
Several patents describe processes wherein fructose is allowed to selectively crystallize from an aqueous solution. In Japanese application 118,200, two towers, one for graining and one for crystallization are described. Feed from the first tower, containing 33-50% fructose syrup, is mixed with massecuite (crystal-containing) overflow from the second tower. The resultant mixture is cooled as the product moves downward in laminar flow. The crystalline fructose is then obtained by centrifugation. Although this process avoids the additional drying and grinding steps of other crystallization processes, its productivity is low and the scale up capacity is limited because of the necessity for vertical laminar flow and heat transfer demands.
One effective procedure for crystallization of fructose from aqueous solutions is described in U.S. Pat. No. 3,928,062.
The patent described a method wherein a supersaturated solution is seeded and then evaporated and/or cooled under moderate stirring while maintaining the concentration and temperature within certain ranges. By continuously concentrating the mother liquor, it can be used to produce multiple crops of fructose crystals. Although a suggestion is also made that cooling alone can be used, such a procedure is not considered as advantageous as those using continuous evaporation because the mother liquid must be reconcentrated at the start of each batch. Although such a procedure is useful for producing small batches of crystalline fructose, such a process could not be used in an industrial scale production due to heat transfer constraints as well as lack of adequate mixing and control of supersaturation.
According to U.S. Pat. No. 3,883,365, large fructose crystals are obtained in a two stage batch method from water solution by adjusting the pH of the solution and slowly cooling the mass to create a supersaturated solution which, when seeded, is optimal for crystal formation. Because of the long crystallization time of the process, a pH adjustment must be done and the productivity of the method is only about 0.25 t/m.sup.3 /d.
Although all of the above processes have been used successfully for the production of crystalline fructose, it has heretofore been thought to be impossible to produce crystalline fructose on a large scale with high yields, high capacity (productivity) and good purity from its aqueous solutions without resorting to costly processing steps including evaporating, drying, and grinding. An object of the present invention is to provide a cost effective method for large scale, high capacity production of fructose crystals in high yields.
Another object of the invention is to provide a method for crystallization of fructose which does not require the use of organic solvents and without the need of pH adjustment.
Still another object of the invention is to provide a crystallizing apparatus that has optimal heat transfer and mixing capacities for large scale production of high purity fructose crystals.
Further objects will be evident from the description of the invention which follows.